1. Field of the Invention
The present invention relates to an apparatus and method for producing blown tubular films from thermoplastic materials. More particularly, this invention relates to improved tube forming and cooling procedures wherein a blown tube is expanded in a prearranged conical restriction and formed by differential air pressure flow produced by cooling air of appropriate pressure directed toward the external surfaces of the advancing tube as it is being formed.
2. Description of the Prior Art
Generally, the prior art technique for forming tubular thermoplastic films comprises continuously extruding a melt of a thermoplastic material through an annular orifice, applying internal fluid pressure to the semi-fluid tube thus extruded and shape expanding the tube to reduce its wall thickness to appropriate dimensions while cooling and solidifying the extruded thermoplastic. Thereafter, the formed tubing is recovered as by passing it over cooling rollers and/or a part of counter-rotating pinch rolls. The flattened tubing may be subsequently passed to a roll forming or wind up turret or directly to further processing as a bag-making operation.
Although useful thermoplastic film tubing has been commercially prepared, under certain circumstances, such a product may have an undesirable gauge non-uniformits, i.e., the thickness of the film is not uniform. Furthermore, the film is not uniformly stressed in a linear and radial direction. Such non-uniform wall thickness results in, for a given average thickness, low gauge points which introduce weak areas in the film. Also, gauge variation results in an uneven, humped roll of film upon winding of the flattened tubing. In addition to the unsightly appearance of such rolls, when the film on such rolls is unwound, it does not lie flat and thus requires special precautions in the printing, conversion and other uses thereof.
One of the major problems in this art is to control cool and control expand the extruded bubble of thermoplastic material at high rates of production. Production rate for any given tube (bubble) size is limited by the character of the bubble being extruded. Thus, under a given set of operating conditions, increasing extruder output will cause the thermoplastic to be formed into the tube at a higher rate but since the heat exchange character of the system will not have changed, it will also cause a rise in the height of the bubble frost line (that is, the line where the extruded and expanding bubble or tube turns from a molten to solid character). This in turn causes an increase in the instability of the extruded bubble because its unsupported molten length has become too long. Supporting the expanding film bubble through a controlled cooling zone in general permits increased extrusion speeds.
The shape of the bubble of cooling thermoplastic film is an important factor in the effect on the final film of variations in the annular die. As might be expected, it is practically impossible to manufacture and maintain a die of exactly uniform width of die gap throughout the circumference thereof. Some adjustment is possible by movement of the inner and outer die faces relative to each other but precision machining of such precise nature as to eliminate significant variation around the die is not now achievable in commercially practicable operations. The variations in gauge as extruded are exaggerated as the bubble of molten film expands and cools.
In the "neck" of the bubble adjacent the extruder die, the resin is molten and hence of low tensile strength. Portions which are of low gauge because of die conformation will be stretched in the neck region to a greater extent than portions of higher gauge, thus increasing the difference in gauge and imparting a different degree of orientation to different areas of the film being formed. This effect increases the gauge variation imposed by the die.
As the film advances in the expanding bubble, it cools and becomes more viscous and stronger as it approaches and passes through the solidification temperature. It will be readily understood that thinner areas cool more rapidly than areas of greater gauge and hence achieve a higher tensile strength. Expansion of the bubble at this stage will cause greater stretch of the thicker portions of higher temperature and thus reduce gauge of these thicker portions to a greater degree than the reduction of the gauge of thinner portions. This effect tends to compensate for inequalities of gauge but can overcompensate to a degree such that the gauge inequalities are reversed and the problem is not really solved.
It has been found that the bubble can be formed and cooled under controlled conditions to exploit the effects described above by causing expansion of the bubble according to a pattern such that differences in tensile strength are caused to act for minimizing of gauge differences resulting from inequalities of the die. This is achieved by imposing on the bubble a shape such that differentials in attenuation of film in the bubble will act at a region of the bubble and to a controlled extent such that gauge variations at the die are compensated but not overcompensated. The bubble shape to achieve this result will vary with nature of the resin, draw down ratio of final film thickness to die gap, rate of production and other process variables.
A system for controlled shape imposition and controlled cooling is described in my prior patents typified by U.S. Pat. No. 3,867,083, dated Feb. 8, 1975. That system provides a plurality of shape imposing cooling rings arranged to provide an inner contour corresponding to desired shape of the bubble. At each ring, two streams of cooling gas, such as air, are ejected to diverge from each other and flow against the bubble of film between such and a face of the cooling ring parallel to the film. The Bernoulli effect causes a vacuum to be created in the space of increased gas velocity between the ring face and the film, thus causing the film to follow the contour of the stack of cooling rings.
That such system of shape imposition and controlled cooling is effective for the purpose has been established by operation on high speed extrusion lines producing film of improved properties. Although the technique is very effective, it has been found that scans of the gauge variation across the film vary from scan to scan in a manner to indicate that, although gauge uniformity has been improved over previous practice, it is still subject to further perfection. It is now proposed that the variation from scan to scan arises from flutter of the film adjacent the faces of the shape imposing rings. While not wishing to be bound by any theory, it is postulated that, as film moves closer to the ring face under influence of reduced pressure due to Bernoulli effect, the space between film and ring face is reduced enhancing the Bernoulli effect and further reducing pressure (increased vacuum) tending to draw the film even closer until flow of air is drastically curtailed, thus building up pressure at the air outlet and forcing film away from the ring face at the point of increased pressure. Film which has approached the face under reduced pressure is then whipped away by the bulge near the air outlet and the cycle repeats. The resultant "flutter" would possibly explain the scan-to-scan variation. Whatever the proper explanation may be, that variation is eliminated while retaining advantages of the vacuum system, by novel means and methods described hereinafter.